The Autonomous Vehicles Systems and Instrumentation Laboratory (AVSIL)
is a facility within the Mechanical and Aerospace Engineering Department
at UCLA. It is designed to allow the evaluation in a realistic environment
of flight hardware for multiple cooperating vehicles. The facility is
centered on a GPS Satellite Constellation Simulator (SCS), which provides
in real time the GPS signals that would be received by a pair of vehicles
in motion at any specified position and velocity. These signals include
both L1 and L2 wavelengths. While many applications require only L1, both
are needed for all current GPS carrier-phase integer ambiguity resolution
algorithms.

In the AVSIL, each of a pair of powerful workstations hosts a high-fidelity
simulation of one vehicle. In the past, these vehicles have been F-18
fighter craft and commercial aircraft, among others. The workstations
compute the dynamics and response of the vehicles in faster than real
time, and send the positions, velocities, and other state and derivate
information to the SCS. The SCS then updates the motion profile of the
vehicles, and produces the R/F signals that would be received by GPS receivers
with antennas mounted at defined positions on the vehicles. The output
of the SCS is plugged directly into the antenna receptacle on the receiver;
no modification of the receiver is necessary.

The workstations are also linked to a device which produces the analog
voltage signals that would be generated by inertial measurement units
(IMUs) on each aircraft. These signals are synchronous with the GPS signals
generated by the SCS, and can be fed directly into the inputs on a flight
control system. In this way, a flight control system can be made to behave
precisely as though it were actually in flight. Because the outputs of
the IMU simulator and the SCS are perfectly known, however, it is possible
to know precisely any errors in the state estimation of the flight control
system. With minor modifications and extensions, the facilities of the
AVSIL can be used to simulate the outputs from a wide variety of instruments.

The AVSIL has been extensively and successfully used in the Autonomous
Formation Flight project at UCLA. In this project, each of two Formation
Flight Instrumentation Systems (FFIS) was connected to the SCS and IMU
simulator. The FFIS units communicate with each other via a wireless link,
and blend Differential Carrier Phase GPS with inertial measurements to
estimate their relative position to within a few centimeters, and other
states to similar levels of accuracy. This is impossible to test in flight,
as no other instruments exist of equal accuracy (that can be used without
extreme expense and restrictions on the tests). In the AVSIL, however,
it is possible to know exactly what the relative states should have been.
It is important that the FFIS units did not have to be modified in any
way to be tested; in this way, it is possible to test exactly the flight
configuration of the instruments.